Electrochemical sensors for oxygen based on the Clark principle (Trans. Am. Soc. Inter. Organs, 2 41 (1956)), in which an oxygen permeable hydrophobic membrane separates an electrolyte-filled chamber containing working and reference electrodes from a liquid to be monitored, have been known for over twenty-five years. In these sensors, oxygen diffusing through the membrane to a platinum cathode is reduced and the current which flows as a result of this reduction is proportional to and is used as a measure of the concentration of oxygen in the liquid.
The signal generated by such sensors decays erratically during continuing use. That is, when sufficient current passes to cause the reference electrode to become polarized, the potential of the platinum electrode to which it is referenced is altered and/or positively charged ions from the reference electrodes migrate to the surface of the cathode rendering that surface ineffective for rapid oxygen reduction. This precludes long term or continuous use of the sensor except in situations where recalibration or compensation is possible.
Difficulties due to electrode polarization and ion migration have been reduced by means of a three electrode cell configuration used with a potentiostat electronic circuit, such as those described in U.S. patent to Galway et al, U.S. Pat. No. 4,227,998 dated Oct. 14, 1980 or U.S. patent to Stretter et al, U.S. Pat. No. 4,326,927 dated Apr. 27, 1982. In these arrangements, a working oxygen electrode, a potential reference electrode, and an indifferent counter electrode are connected in a circuit providing a high input impedance, e.g. one million ohms at the reference electrode so that only very small current, e.g. one billionth of an ampere or less can pass through the reference electrode and the potential of the reference electrode is not likely to drop or become polarized. Since the reference electrode is no longer a significant current pathway, a counter or auxiliary electrode is provided through which the oxygen dependent current passes. The potentiostat measures the voltage difference between the working electrode and the reference electrode, compares that to the desired voltage for rapid oxygen reduction and drives the current between the working electrode and the counter electrode until the voltage between the working and reference electrodes is as desired.
While the three electrode potentiostat system reduces difficulties due to polarization and ion migration, previously known electrochemical sensors have remained subject to lack of stability, erratic signal generation, and the development of electrode surface areas poisoned by adsorbed materials.